This invention relates to well pump assemblies for pumping groundwater. More particularly, this invention relates to well screens, strainers, and filters for use in water wells.
Deteriorating well performance commonly results from failure of screening surrounding a pump inlet positioned in a bore of a well. Screen failure can often be attributed to blockage of the screen by chemical incrustation, biofouling, or physical blockage by silt, sand or gravel. Alternatively, failure can result from corrosion of the screen (either mechanical or chemical) which decreases well performance by widening the screen pores enough to allow passage of sand through the screen. The resultant influx of sand particles into the pump can greatly reduce pump life and efficiency.
One method of increasing the usable lifetime of screens involves centralizing the screen in the bore of the well and packing gravel around the screen. The gravel, commonly known as a "gravel pack", acts as a prefilter that traps sand particles before they can clog or corrode the screen. One example of such a gravel pack is discussed in U.S. Pat. No. 1,619,521 to Lawlor et al., issued Mar. 1, 1927. Another design for a well screen is found in U.S. Pat. No. 2,266,000 to Charles, issued Dec. 16, 1941. Both of these screen designs (or "strainers" as they are often called in the literature) utilize gravel as a supplementary screening medium that retards the passage of sand toward the pump inlet.
However, gravel packs are not an ideal solution to the problem of screen clogging or corrosion. Although clogging or chemical incrustation of the gravel pack takes longer than clogging or incrustation of the screen, eventually the gravel pack will become clogged with particles, or biologic or chemical incrustations. Even replacement of a clogged gravel pack is not a completely satisfactory solution because the changes in velocity flow caused by the transition between the gravel pack and the native rock and gravel formations tends to induce chemical precipitation in the native formations surrounding the gravel pack. Eventually this chemical precipitation (or particle or biologic clogging) will block pores in the native formation, diminishing its permeability to groundwater flow. Even if the gravel pack is replaced, the well performance can be permanently reduced by blockage of the pores in those regions of the native formations surrounding the gravel pack.
The present invention encompasses an assembly for pumping groundwater from native formations that includes pumping means for pumping groundwater. The pumping means has an inlet positioned in a bore of a well for intake of water contained in groundwater bearing native formations and screening means for screening the inlet to prevent intake of particles greater than a predetermined size. A gravel pack is positioned in the bore of the well to surround the screening means and filter groundwater flowing toward the pump inlet, collecting particles (such as sand and silt) and chemical deposits precipitated within the gravel pack by changes in flow velocity of groundwater. Means for barring collapse of native formations into the bore of the well following removal of the gravel pack, the screening means, and the inlet from the bore of the well are also provided.
In preferred embodiments, means for barring collapse of native formations into the bore of the well includes a slide tube dimensioned to fit within the bore of the well between the native formations and the gravel pack. When it is necessary to remove the gravel pack for replacement with fresh gravel, the slide tube is inserted down the bore of the well to hold back collapse of native formation material into the bore of the well as the gravel pack is removed. Following replacement of the gravel pack, the slide tube is withdrawn from the bore of the well.
In another embodiment, means for barring collapse of native formations into the bore of the well include a porous casing permanently positioned in the bore of the well in contact with native formations. The porous casing is configured to have a plurality of pores therethrough, with pore sizes dimensioned to present substantially no impedance to flow velocity of groundwater moving through the porous casing toward the pump inlet. In this embodiment of the invention, groundwater flowing toward the pump inlet moves without substantial velocity changes from native formations, through the porous casing, and into a gravel pack positioned between the screening means and the porous casing.
The present invention also provides an assembly for pumping groundwater from native rock and gravel formations. The pump assembly has a pump and a pump inlet positioned in the bore of the well. The pump inlet is surrounded by a screen configured to have a plurality of apertures therethrough. The apertures are sized to prevent intake into the pump inlet sand-sized or larger of particles
The porous casing has a plurality of pores with pore sizes substantially greater than the aperture size of the screen. In addition, the pores of the porous casing are dimensioned to present substantially no impedance to flow velocity of groundwater moving through the porous casing toward the pump inlet. Groundwater flowing toward the pump inlet easily moves without substantial changes in flow velocity through the porous casing and into a gravel pack positioned between the screen and the porous casing.
The gravel pack contains gravel having a gravel size greater than the aperture size of the screen. The integral pores extending through the gravel pack are selected to present pore diameters substantially corresponding to the pore diameters of gravel or rock in native formations. Because of the matched pore sizes of the gravel pack and the native formations, the flow velocity of groundwater does not substantially change as groundwater flows from the native formations into the gravel pack. Deposition of chemical precipitants and particles is promoted in the gravel pack, not in the surrounding native formations, since the velocity of groundwater flow changes only within the gravel pack near the screen (which has much smaller apertures sized to prevent influx of gravel and sand from the pack into the pump inlet).
When well groundwater production begins to decrease, it is only necessary to withdraw and replace the "used" gravel pack and its associated contaminants with a fresh gravel pack to bring the well back to normal production level. Because of this design, little or no deposition of particles or chemical incrustations occur in the native formations surrounding the bore, since during operation of the well there was no substantial reduction in the velocity of groundwater flow in the native formations that trigger such depositions of contaminants.
In other preferred embodiments, an impervious casing is positioned in the bore of the well to extend downward to meet the porous casing. Sealing means are provided to seal the bore between the pump inlet and the impervious casing. The sealing means prevents fluid communication between portions of the bore above the sealing means and portions of the bore below the sealing means. The sealing means includes an annular member formed from gas impermeable elastic material that can be inflated with gas to expand the annular member about the tube and form a seal between the tube and the impervious casing. Deflation of the annular member by gas release serves to break the seal. To aid in determining the extent of the clogging of the gravel pack, it is possible to position a pressure sensor below the annular member to measure changes in fluid pressure. Significant decreases in fluid pressure signal the need to replace the gravel pack.
Additional features and advantages of the invention will become apparent to those skilled in the art on consideration of the following detailed description of preferred embodiments exemplifying the best mode of carrying out the invention as presently perceived.